[ntp:questions] Clock and Network Simulator
David L. Mills
mills at udel.edu
Wed Jun 30 22:00:06 UTC 2010
Is there somebody around here that understands feedback control theory?
You are doing extreme violence to determine a really simple thing, the
discipline loop impulse response. There is a much simpler way.
Forget everything except the tools that come with the NTP distribution.
Find a good, stable server and light up a selected client. Make sure the
client kernel is enabled. Set minpoll and maxpoll to 6. Configure the
loopstats monitoring function. Run the client until operation stabilizes
as determined by the loopstats data
While the daemon is running, use ntptime to set the clock offset to 100
ms. Go away and do something useful a couple of yours.
Inspect the loopstats data. It should start at 100 ms, exponentially
decay to zero in about 3000 s, overshoot about six percent of the
initial offset., then slowly decrease to zero over a period of hours.
This is the intended nominal behavior for a poll interval (same as time
constant) of 6. If you increase (decrease) the poll interval by one, the
impulse response will look the same, but at double (half) the time
scale. This should hold true for poll intervals from 3 to 10.
Of particular importance is the damping factor, which is evident from
the overshoot. If SHIFT_PLL is radically changed, I would expect the
overshoot to be replaced by an exponentially decaying ring
characteristic. With the intended loop constants the behavior should
have a single overshoot characteristic in the order of a few percent.
From a mathematical and engineering point of view the intended behavior
provides the fastest convergence, relative to the chosen time constant,
with only nominal overshoot.
If the intended effect of the SHIFT_PLL change was to decrease the
convergence time, that is the absolute worst thing to do. The nanokernel
allows the time constant to range from zero to ten and carefully scales
the state variables to match, although it (and the daemon discipline)
starts to become unstable at values below 3, the minimum enforced by the
daemon. The change in SHIFT_PLL would result in unstable behavior below
5 (32 s), as well as serious transients if the discipline shifts from
the daemon to the kernel and back. All feedback loops become unstable
unless the time constant is at least several times the frequency update
interval, which is this case is one second. If you do want to explore
how stability may be affected, restore the original design and recompile
the distribution with NTP_MINPOLL changed from 3 to 1.
Now to the issue of multiple tandem server/clients. You don't need to
explore the behavior; it can be reliably predicted. Assume the server
and all downstream clients are started at the same time. The impulse
response of the first downstream client of the original client
operating as a server is the convolution of the original impulse
response with itself. Roughly speaking, the offset decay is slower,
reaching zero in twice the original time or about 6000 s. The behavior
of the next downstream client is the convolution of this convolution and
the original impulse response and so on.
To fix the original problem reported to me, change the frequency gain
(only) by the square of 100 divided by the new clock frequency in Hz.
For instance, to preserve the loop dynamics with a 1000-Hz clock, divide
the frequency gain parameter by 100. In the original nanokernel routine
ktime.c at line 60 there is a line
SHIFT_PLL * 2 + time_constant.
Replacing it by SHIFT_PLL * 20 + time_constant would fix the progblem
for 1000-Hz clocks.
Miroslav Lichvar wrote:
>On Tue, Jun 29, 2010 at 06:31:01PM +0000, David L. Mills wrote:
>>From your description your simulator is designed to do something
>>else, but what else is not clear from your messages. It might help
>>to describe an experiment using your simulator and show what results
>It's designed to test NTP implementations, but it uses a more general
>Ntpdsim tests ntpd as an NTP client with simulated NTP servers in a
>simulated network. Clknetsim doesn't simulate NTP servers, it
>simulates only a network to which are connected real NTP clients and
>The difference is that ntpdsim tests one NTP client and clknetsim
>tests whole NTP network.
>Say we want to test how does the Linux SHIFT_PLL change affect an NTP
>network. There is a chain of seven ntpd daemons configured, all using
>poll 6. Strata 1, 3, 5, 7 have SHIFT_PLL 2 and strata 2, 4, 6 have
>SHIFT_PLL 4. Stratum 1 has clock with zero wander and frequency offset
>and is using the LOCAL driver, the rest have clocks with 1ppb/s
>wander. Between all nodes is network delay with exponential
>distribution and a constant jitter. The simulations are repeated with
>jitter starting from 10 microseconds and increased to 0.1 second in 28
>steps. Each simulation is 4000000 seconds long and the result is a
>list of RMS offsets, one for each stratum.
>After finishing all iterations, we'll make an RMS offset/jitter plot:
>And the same experiment with all strata using SHIFT_PLL 4:
>You probably know what to expect here, but I was surprised to see that
>with high jitter the SHIFT_PLL 4 strata are actually better than their
>SHIFT_PLL 2 sources.
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